This invention relates broadly to neutron monochromators and more particularly to pulsed neutron monochromators for use in time-of-flight neutron spectrometry.
Time-of-flight neutron spectrometers are utilized in various research applications, such as in elastic-neutron-scattering analyses for providing valuable information on dynamic properties of materials. A time-of-flight spectrometer requires a pulsed neutron monochromator for producing pulses of essentially monoenergetic neutrons. Preferably, the monochromators should be capable of producing pulses at a controlled, high repetition rate, the pulses being characterized by both high intensity and very short duration.
The prior art includes mechanical neutron choppers consisting of one or more high-speed rotary discs having apertures for pulsing, or chopping, a neutron beam. Such devices are relatively expensive to construct and maintain; furthermore, they are limited with respect to changing pulse rate or pulse duration. A pulsed-neutron monochromator utilizing a ferrite crystal and a magnetic drive coil therefor is described in U.S. Pat. No. 3,517,193 (June 23, 1970; H. A. Mook et al). That monochromator is subject to some limitations imposed by the small size of ferrite crystals.
The prior art also includes various pulsed-neutron monochromators which utilize nearly perfect single crystals of silicon, silicon dioxide, quartz, and the like. These monochromators are not well suited for time-of-flight spectrometry because they do not generate sufficiently short neutron pulses. In some monochromators, a row of crystals is disposed in a neutron beam, with the crystals positioned to reflect continuous beams of neutrons onto a common target. The various crystals are oriented to define increasingly large scattering angles throughout the row in order to increase the intensity of the reflected beams. Such monochromators are incapable of distinguishing between elastically and inelastically scattered neutrons.